10.1 Introduction
The fortunes of Homo sapiens, once a small insignificant population of a mediumsized mammal, changed fundamentally with the domestication of animals and
cultivation of crops. These transitions kick-started massive population growth and
increased further spread of humans around the world (Bocquet-Appel 2011;
MacHugh et al. 2017). Human migration often went hand-in-hand with the migration of domesticated animals, and today it is estimated that there are globally
approximately 7 billion humans, a billion sheep, a billion pigs, more than a billion
cattle, 25 billion chickens, and millions of horses and donkeys (Wolfe et al. 2007;
Harari 2015). These animals have been selectively bred for traits that humans found
desirable e.g., milk, meat, eggs or wool production, for transport, and to serve as
draught animals. This approach has resulted in decreased genetic diversity across
these domestic species, which often leads to less resilience and greater vulnerability
to pathogens (Gunderson et al. 1995). In the context of what is discussed below, this
has enormous relevance to these species and others as hosts of infectious diseases.
A species is never introduced to a new area alone. They are in fact biological
packages, because many microbes and viruses inhabit the larger species that act as
their hosts. The movement of animals from place to place, therefore, implies the
movement of all microscopic passengers that they are hosting. Some of these
microbes are necessary for the survival of the animal; for instance, microorganisms
in the gut of ruminants allow their hosts to digest their cellulose-rich food, while
others are commensals or pathogens (Bergmann 2017).
The expansion of these populations has meant that the number of hosts for
diseases of these species and their relatives has expanded massively along with
exposure to new diseases from invasion of wildlands and subjugation of these for
anthropological use (Tilman and Lehman 2001). This has meant close contact
between humans, their domestic stock and wildlife (Acevedo-Whitehouse and
Duffus 2009). The interface between these is an ideal venue for transmission of
infectious diseases in many directions (Deem et al. 2001; Pearce-Duvet 2006). We
can envisage transmission from wildlife to livestock, or from stock to wildlife, or
humans to livestock (anthropozoonotic) and then wildlife, or vice versa, i.e. animals
to humans (zoonotic).
Since parasites generally cause harm to their hosts, the infectious diseases we
refer to here can for practical purposes be considered parasites. The effects of
parasites in an ecosystem are diverse, as described by Hatcher et al. (2012). The
most obvious effect is the direct harm caused by parasites to their hosts. Individual
hosts can be killed, or their ability to survive and reproduce otherwise directly
reduced, which in turn reduces population numbers. Individuals infected by a
parasite may also show a change in behaviour. A combination of these effects can
change the social structure and ecology of the affected population. A disease could
even cause the extinction of a particularly vulnerable species. Infectious disease has
been recorded as contributing to the demise of 4% of extinct species, and the
critically endangered status of 8% of species classified as such by the International
250
L. van Helden et al.
The fortunes of Homo sapiens, once a small insignificant population of a mediumsized mammal, changed fundamentally with the domestication of animals and
cultivation of crops. These transitions kick-started massive population growth and
increased further spread of humans around the world (Bocquet-Appel 2011;
MacHugh et al. 2017). Human migration often went hand-in-hand with the migration of domesticated animals, and today it is estimated that there are globally
approximately 7 billion humans, a billion sheep, a billion pigs, more than a billion
cattle, 25 billion chickens, and millions of horses and donkeys (Wolfe et al. 2007;
Harari 2015). These animals have been selectively bred for traits that humans found
desirable e.g., milk, meat, eggs or wool production, for transport, and to serve as
draught animals. This approach has resulted in decreased genetic diversity across
these domestic species, which often leads to less resilience and greater vulnerability
to pathogens (Gunderson et al. 1995). In the context of what is discussed below, this
has enormous relevance to these species and others as hosts of infectious diseases.
A species is never introduced to a new area alone. They are in fact biological
packages, because many microbes and viruses inhabit the larger species that act as
their hosts. The movement of animals from place to place, therefore, implies the
movement of all microscopic passengers that they are hosting. Some of these
microbes are necessary for the survival of the animal; for instance, microorganisms
in the gut of ruminants allow their hosts to digest their cellulose-rich food, while
others are commensals or pathogens (Bergmann 2017).
The expansion of these populations has meant that the number of hosts for
diseases of these species and their relatives has expanded massively along with
exposure to new diseases from invasion of wildlands and subjugation of these for
anthropological use (Tilman and Lehman 2001). This has meant close contact
between humans, their domestic stock and wildlife (Acevedo-Whitehouse and
Duffus 2009). The interface between these is an ideal venue for transmission of
infectious diseases in many directions (Deem et al. 2001; Pearce-Duvet 2006). We
can envisage transmission from wildlife to livestock, or from stock to wildlife, or
humans to livestock (anthropozoonotic) and then wildlife, or vice versa, i.e. animals
to humans (zoonotic).
Since parasites generally cause harm to their hosts, the infectious diseases we
refer to here can for practical purposes be considered parasites. The effects of
parasites in an ecosystem are diverse, as described by Hatcher et al. (2012). The
most obvious effect is the direct harm caused by parasites to their hosts. Individual
hosts can be killed, or their ability to survive and reproduce otherwise directly
reduced, which in turn reduces population numbers. Individuals infected by a
parasite may also show a change in behaviour. A combination of these effects can
change the social structure and ecology of the affected population. A disease could
even cause the extinction of a particularly vulnerable species. Infectious disease has
been recorded as contributing to the demise of 4% of extinct species, and the
critically endangered status of 8% of species classified as such by the International
250
L. van Helden et al.